Concussions are increasingly being diagnosed in kids — especially those participating in contact sports like football — and new research shows that having more than one concussion can prolong a child’s recovery.

According to a new study published in the journal Pediatrics, kids who experienced a second concussion within a year of their first head injury suffered symptoms for almost three times as long, on average, as children whose concussions were spaced more than a year apart. Kids over age 13 tended to have longer recovery times than younger children.

About 144,000 children go to the emergency department each year with a concussion, and millions of others are treated by primary-care doctors, athletic trainers and specialists. To assess the risk of multiple concussions, researchers from Boston Children’s Hospital and Harvard Medical School looked at 280 study participants from ages 11 to 22 who were treated in the emergency department for concussions. They analyzed whether the children’s previous concussions had an effect on their recovery from further head trauma.

Previous research in animals has reached similar conclusions: symptoms seem to last longer after multiple head traumas. The new research supports these findings and is the first to demonstrate the results in humans. “This has direct implications on the management of athletes and other at-risk individuals who sustain concussions, supporting the concept that sufficient time to recover from a concussion may improve long-term outcomes,” the authors write.

What’s worrying about the effects of multiple concussions in adolescents is that other studies have shown athletes — including those in high school — tend to not reveal when they’re experiencing concussion symptoms. Research presented at the Pediatric Academic Societies’ annual meeting in Washington, D.C., last month showed that despite the fact that high school football players are aware of the risks associated with concussions, a little over half of 120 high school players surveyed said they would still play through a head injury.

The authors of the Pediatrics study say their findings are especially relevant for athletes, who have a greater likelihood of experiencing concussions, and multiple ones at that. As research continues, increased awareness about the ramifications of playing with a concussion will allow coaches and players to become more knowledgeable about the risks — and more apt to put aside pride to sit on the sidelines if necessary.